The present invention relates to electrical heaters with resistive wire heating elements, and more particularly to an electric heater with multiple resistive wire heating elements in an inner sheath, said inner sheath being packed into an outer tubing.
It has been known for some thirty years or more in the heater industry to make heaters having a single outer tubing in which is placed multiple sheathed heaters, each sheathed heater having a resistive coiled wire as a heating element. The resulting heater is known in the industry as a xe2x80x9cmulticellxe2x80x9d because it has multiple heaters wrapped in a single tubing. In general the concept of multicell heaters are describe in U.S. Pat. No. 3,340,382 to Lennox, which is incorporated herein be reference.
The outer tubing (sometimes referred to as an xe2x80x9coversheathxe2x80x9d) provides extra protection from the elements by further isolating the heating elements from possible contaminants. The outer tubing and can also provide the ability to place the heating power of several heaters within a body having a regular (cylindrical) contour, thus allowing it to be used like a cartridge heater, for example in heating large platens for use in molding aircraft parts. Having multiple heating elements within the same outer sheath allows for heating elements made of larger gauge wire without a reduction in total heat flux (sometimes referred to as xe2x80x9cwatt densityxe2x80x9d). This thicker wire typically has a longer life than heating elements made of a narrow gauge wire.
Another example of a heater with outer tubing can be found in U.S. Pat. No. 5,575,941 to Johnson, which is incorporated herein by reference. Johnson attempts to use outer tubing with a single sheathed heating element folded over several times within the outer tubing. The result is a cylindrical heater (like a cartridge heater) with a similar effect to having four heating elements within. Because the single heating element is folded over, it appears at any given perpendicular cross-section of the heater (except at the ends) that it has four heating elements.
xe2x80x9cLike prior art, multicell heaters, Johnson has several limitations. The most important of which is that, like prior art multicell heaters, the toe (end) of the heater is left colder than the rest of the outer tubing. At the end of the outer tubing, the inner heater in Johnson only makes contact at a single point (arguably two single points, however in close proximity to one another). Furthermore, the construction of the heater in Johnson does not leave any additional space within the outer tubing for additional components, such as a thermocouple or other type of sensor.xe2x80x9d
It is thus an object of the present invention to provide a multicell heater with only a single inner sheath within the outer tubing.
It is also an object of the present invention to provide a multicell heater with a toe end that heats to approximately the same temperature as the sides of the outer tubing.
It is yet a further object of the present invention to provide this multicell heater with a thermal well wherein optional sensors may be easily placed.
In keeping with the above, the present invention comprises outer tubing, wherein a plurality of resistive heating elements having a common sheath are placed. In the preferred embodiment, the resistive heating elements are parallel and coplanar within the inner before insertion into the outer tubing. The inner sheath is then bent into a xe2x80x9cUxe2x80x9d shape with the bend being made perpendicular to the direction of the heating elements. Thus, once inserted into the outer tubing, the inner sheath has a complete line of contact with the end disk of the outer tubing, rather than a single point of contact. The outer tubing will usually be round (cylindrical) to accommodate most practical applications, however the invention could work just as easily with outer tubing having other cross-sections should the need arise.
To produce this configuration, the inner sheath preferably begins with a non-circular cross-section. A typical cross-section for the inner sheath has flattened sides with semi-circular ends as will be made apparent below.
The bend in the inner sheath provides a gap within the outer tubing which may be filled with reinforcing bars, hollow tubes, or a combination thereof. The hollow tubes may be used as thermal wells, in which sensors such as thermocouples may be positioned.
A typical application for the multicell heaters of the present invention would be to provide a plurality thereof to be inserted together into a superplastic forming platen. Another application would be to place a plurality of the heaters in a duct to heat air or other gasses by forced convection. The number of different applications for the present heater is infinite. It can be used in any manner conventional multicell heaters are used, but are less expensive to manufacture and provides more efficient heating at the toe end.